The present invention relates generally to methods and circuits for transmission of direct-current, and more particularly to a method and a circuit for transmission of DC current between at least one rectifier and at least one inverter.
DC transmission systems are often used to transmit electric energy between two non-synchronizable alternating voltage power lines use. DE-AS 25 18 910 discloses a DC transmission system, in which a rectifier is connected to the one alternating voltage power line. The rectifier produces a DC current which is fed to an inverter. The latter, in turn, is connected to the second alternating voltage power line. The two alternating voltage power lines are then coupled to each other via a DC connection.
Various regulating methods are used to operate such DC transmission systems. A large number of customary methods of regulation are based on the marginal current method. In the marginal current method, both rectifier and inverter have a current regulator, in which the characteristic curves are shifted from each other by a marginal current. In addition, in this method the inverter has a so-called extinction angle regulator. EP O 197 352 A1 discloses a circuit which has a current regulator for the converter present in rectifier operation and an extinction angle regulator for the converter which is present in inverter operation. The current regulator of the rectifier receives as input signal the desired current value Id*. Id* and the extinction angle value .gamma. are produced by a control and calculation member to which Udn, Ido, P, P* and .gamma. (extinction angle signal of the inverter) are fed.
In the marginal current method, during steady-state operation the rectifier determines the DC current and the inverter determines the DC voltage on the transmission path. Considered from the standpoint of the DC regulating circuit of the rectifier, a disturbance in the power line (decrease or increase of the three-phase current voltage) on the inverter can be amplified by the extinction angle regulator. Furthermore, considered from the standpoint of the inverter-side power line, the extinction angle regulator has a poor du/dq behavior since it does not counteract a change in three-phase voltage. In order to eliminate this disadvantageous behavior of the extinction angle regulator in connection with the marginal current method, DC voltage regulators are used which have the desired behavior upon increases in the three-phase voltage. The DC voltage regulators, however, impair the efficiency of the DC transmission system.
The present invention is directed to the problem of developing method and circuit for transmitting DC current in which the above-mentioned disadvantages are avoided.